The well known pasteurization process to kill bacteria in milk has been used for many decades. Unfortunately, the higher temperatures needed in the pasteurization process adversely affect the flavor of the milk. Further, even with the use of such high temperatures, the pasteurization process does not eliminate all undesirable bacteria, leading to the short storage stability of most milk products.
Bacillus cereus are often the predominant bacteria in conventionally processed milk of relatively advanced age, because they can survive the pasteurization process and they thrive at cold temperatures, promoting the spoilage of the milk. A general need exists for a method for reducing the content of bacteria in milk, both whole and skim milk, to enhance the storage stability of the product and to improve its flavor by elimination of the pasteurization process.
Of great economic importance, also, is the need to eliminate the very expensive and laborious distribution method that is now necessary to place milk in the hands of the consumer. It is now necessary for every dairy, after processing of the raw milk through homogenization and other steps, to fill the milk into containers for distribution to the consumers and to transport that milk under refrigerated conditions. This requires every dairy to purchase and maintain a significant fleet of refrigerated trucks to transport the processed milk to the point of distribution to the consumer. By providing a sterile or nearly sterile milk product, it would be possible to eliminate the need for transportation of milk under such refrigerated conditions. Unfortunately, the pasteurization process only provides milk with a reduced bacterial count, and not a sterile product.
Further, if a sterile milk product could be produced, it would also be possible to eliminate the need for storing the milk at the point of distribution under refrigerated conditions. Elimination of the need for large refrigerated compartments, as in the typical grocery store, would also be of tremendous economic benefit.
Even when the present day pasteurization process is employed, in some instances it is of particular importance to obtain milk with a lowered bacterial content, prior to pasteurization. For example, a particular batch of raw milk may be so contaminated that mere pasteurization will not result in even adequate storage life by today's standards.
For some applications, moreover, it is of value to be able to provide treated milk in which the bacteria content has been greatly reduced e.g., to about one hundredth of the original value. It is especially important to provide milk with a relatively low bacterial content for the production of cheese, since incorrect bacteria cultures can destroy the cheese. It is normally not suitable to simply heat-treat milk to a sufficient degree for use in cheese production, because such heat treatment may give a lower yield of cheese and can also adversely influence the coagulation time. Conventionally, additives are employed to reduce the problem. In many instances, however, it would be desirable to avoid the use of such additives.
Various methods for producing milk with a lowered bacterial count through the use of filtration have been known in the art, but none have found wide acceptance. The prior art methods generally provide either poor flow rates, rendering the method uneconomical on a large scale, or adversely affect the quality of the milk, making the product unacceptable to consumers.
Conventional filtration means for producing milk with a lowered bacterial content have been attempted. Swedish patent publication No. 380,422 discloses a method in which whole milk is divided into filtrate and concentrate fractions by microfiltration. The filtrate that passes through the pores of the filter (the size of the pores may range broadly from 0.1 micron-10 micron) consists of milk with substantially reduced fat content and the concentrate, which is the fraction retained by the surface of the filter, consists of cream, as not only bacteria, but also fat globules are substantially retained by the filter.
U.S. Pat. No. 5,064,674 relates to a method for making hypoallergenic milk by ultrafiltration methods employing membranes that will allow molecules having a molecular weight of less than or equal to about 5 kDa to pass therethrough. The excluded components that are trapped by the membrane include milk protein, viable or non-viable bacteria, bacterial protein antigen, and milk fat. The filtrate collected from the ultrafiltration process therefor is free not only of bacteria and bacterial protein antigen, but also fat and milk protein, making the product unsuitable for use as milk, per se.
It is clear, then, that the pores of bacteria filters used in the art, which filters are effective to sterilize milk, also will remove not only the bacteria, but also the fat globules, and at least some of the proteins. Such a filter quickly becomes blocked by trapped material; hence, the flow rate through the filter rapidly declines and the filter must be frequently cleaned or replaced. The high cost of such an inefficient process is generally prohibitive. Further, because the filter holds fat globules and proteins, the quality of the milk is also adversely affected.
From the foregoing discussion, it is apparent that there is a continuing need for an improved milk filtration processing method that can provide a sterile, or more nearly sterile product, that has an improved storage life, and does not adversely affect milk quality.
Some attempts have heretofore been made to use cross-flow, or tangential flow, filtration devices to treat milk, such devices being known in the art.
Several types of filtering devices have been described which enable such tangential or crossflow filtration to be accomplished. Perhaps the oldest such apparatus known, described in Soviet Pat. No. 142,626 to Zhevnovatyi, A. I. in 1961, is formed by a tube of porous material fixed inside a second tube, the suspension to be filtered passing under load at high velocity in the annular space between the two tubes, the filtrate flowing within the porous tube. Improved devices of similar construction use two concentric cylinders, with the internal cylinder being formed by a microporous membrane, the liquid being subjected to a forced helicoidal flow around such internal cylinder.
Other crossflow devices comprise a series of filtering elements superposed in the form of plates or disks, on the two faces of which microporous membranes are arranged, for example, around a filtrate-collecting tube, the suspension to be filtered passing between the disks in a helicoidal path one after another.
Many other variations on the crossflow filtration system have been developed. For example, U.S. Pat. No. 5,009,781 relates to a cross-flow filtration device with a filtrate network that includes a number of longitudinal filtrate chambers and one or more filtrate channels which transect the chambers. U.S. Pat. No. 5,035,799 relates to a crossflow filter assembly having filter leaf assemblies arranged in parallel within the filter tank, with pressurized input to create turbulent crossflow of fluid over the media.
U.S. Pat. No. 5,015,397 relates to a crossflow filtration apparatus and process which includes a tube of helically wound wedge wire. Contaminated influent enters at one end and as it flows through the tube, it becomes more concentrated with contaminants, while clarified liquid permeates through the tube wall. U.S. Pat. No. 5,047,154 relates to a method and apparatus for enhancing the flux rate of crossflow filtration systems. U.S. Pat. No. 4,569,759 relates to a tangential filtration apparatus and a plant comprising such an apparatus.
Cross-flow filtration is substantially different from through-flow filtration, in that the liquid feed is introduced parallel to the filter surface, and filtration occurs in a direction perpendicular to the direction of the feed flow. In cross-flow filtration systems, generally, because the direction of the feed flow is tangential to the membrane surface, accumulation of the filtered solids on the filtering medium is reduced by the shearing action of the flow. Cross-flow filtration thus affords the possibility of a quasi-steady state operation with a nearly constant flux when the driving pressure differential is held constant. Unfortunately, this theoretical possibility has not been achieved in practice. Thus, the problem of declining filtration fluxes has plagued conventional cross-flow filtration systems. The majority of the suspended solids are retained on the wall of the tube and quickly form a dynamic membrane (also referred to as a "filter cake" or "sludge layer"). The dynamic membrane is largely responsible for the filtration which subsequently occurs.
Those particles initially entering into the wall matrix ultimately become entrapped within it, because of the irregular and tortuous nature of the pore structure. As microfiltration proceeds, penetration of additional small particles into the wall matrix is inhibited by the presence of the dynamic membrane. The formation of the dynamic membrane, together with the possible clogging of the pore structure of the tube by entrapped particles, results in a decline in the filtration flux. In conventional systems, this decline is approximately exponentially related to filtration time.
Crossflow filtration of milk has been attempted, but has not been generally accepted because of the problems discussed above. U.S. Pat. No. 5,028,436 relates to a process for separating the dissolved and undissolved constituents of milk, using a microporous membrane with a pore size in the range of 0.1 to 2 microns, which has been pretreated with an aqueous solution, dispersion or emulsion of lipids or peptides and the milk separated on the pretreated membrane. In the method of the patent a first filtration step is employed using a microporous membrane in a tangential flow mode. A clear filtrate and a thickly flowing concentrate are obtained. The filtrate contains all salts, lactose, amino acids, oligopeptides and poly-peptides of low molecular weights in genuine, non-denatured form and the concentrate contains practically all casein and fatty components of the milk. Thus, the filtrate cannot be considered to be "milk" as the fatty substances have all been removed therefrom.
U.S. Pat. No. 4,876,100 relates to a crossflow filtration method for producing milk with a lowered bacterial content. Raw milk is divided by centrifugal separation into one fraction consisting of cream and another fraction consisting of skim milk. The skim milk fraction is directed into a microfilter in which part of the fat globules, protein, and bacteria are separated. From the microfilter there is obtained a filtrate which consists of skim milk having a lowered fat, protein and bacterial content, and a concentrate having an increased fat, protein and bacterial content. The concentrate is subsequently sterilized. Thus, the filtration method of the '100 patent, besides reducing bacterial levels in the filtrate, also reduces the fat and protein content of the filtrate, altering its characteristics from that of the original skim milk.
Clearly, the use of crossflow filtration, to date, has not provided an acceptable method for reducing bacterial contamination in milk.
One means to overcome some of the problems associated with classical crossflow filtration technology, known as dynamic microfiltration, has emerged. The dynamic filtration process overcomes the disadvantage in the classical crossflow technology because the liquid to be filtered is not simply guided tangentially over the membrane surface. The membrane surface or a solid body near the membrane surface is moved such that the fluid at the interface between the rotor and the stator is subjected to shearing action. The shearing action tends to "scrub" the membrane surface, keeping it relatively clear of particulate material, and preventing a filter cake from forming on the membrane surface. The particulate material that would otherwise collect on the membrane surface remains suspended, and is ultimately removed in the secondary stream, generally referred to as a concentrate stream.
Dynamic microfiltration systems may take various forms. For example, U.S. Pat. Nos. 5,037,562, 3,997,447, 5,037,562, 3,997,447 and 4,956,102 relate to dynamic microfiltration disc systems.
Cylindrical dynamic microfilters devices are taught in U.S. Pat. Nos. 4,956,102; 4,900,440; 4,427,552; 4,093,552; 4,066,554; and 3,797,662, as well as many others. All patents referenced in the present application are incorporated herein by reference.
No one has ever applied dynamic microfiltration to the processing of milk, and the use of crossflow filtration of milk has been limited, and principally used to fractionate milk into components based upon fat content.